The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

	Lens and the sensitive plate, and Mr. Dallmeyer supplies these negative lenses so as to work either with ordinary doublets or portrait lenses - thus it does not become necessary for the tourist, in all cases, to take with him a complete telephotographic combination in addition to his ordinary lenses. For securing instantaneous pictures of distant or shy animals, however, it-is very desirable to use the telephotographic system in which a portrait lens is the positive combination; and with this some very remarkable pictures of birds and other easily frightened animals have been secured. The Dallmeyer M Stigmatic " lens (fig. 98) is a portrait combination, which appears to have all the advantages, and perhaps more, than the older construction to which the astigmatic corrector is adapted. At the full aperture, spherical aberration is as perfectly corrected as in the best portrait lenses of the old or Petzval type, and a flat field is obtained with a mere trace of astigmatism ; at the same time it includes an angle of 6oo. The above statement will show how well the new lens is adapted for astronomical work or portraiture, and the construction of such a lens must be considered one of the optical triumphs of the time; but perhaps one which will be more appreciated by the astronomical photographer than the ordinary amateur or tourist, to whom a lens working athas but limited uses, owing to. that lack of depth of focus which is intrinsically inherent to such an aperture; but to the professional photographer, or one wishing to obtain studies of animal life, the stigmatic lens gives new powers. The defining power of the stigmatic lens may be reduced by a provision for disturbing the correction of spherical aberration, in which case it gives a uniform softness all over the field. The above remarks apply especially to the stigmatic lens in its original form and working at; but quite recently Mr. Dallmeyer has introduced a sim-plified and portable form (fig. 99), working atalso including a wider angle. This instrument may well claim to be a uni-versal lens, having all the advantages of the recent stigmatic and anastigmatic forms, together with greater rapidity than most of them. The freedom from spherical aberration makes the defining power quite independent of the diaphragm, and there is no shifting of the focus on altering the aperture. The angle in-cluded is about 850 on the diagonal of plate, and this most recent introduction should prove of great advantage to those who wish 438